Here's how it works: Developers place ads on Facebook's website and its mobile apps. When users install their apps, developers pay a fee.

Because Facebook has a huge audience, and because its fast-growing base of mobile users have already shown they're willing to download Facebook's own app, it's a natural audience for these ads. TinyCo, a mobile games maker, reported having 50 percent higher click-through rates. Companies like Zoosk, the online-dating service, and Fab, the e-commerce site, have also been running ads in tests.

Facebook has been swiftly laying the groundwork for this business, first launching its App Center, a centralized place for promoting apps, in May and rolling it out internationally.

Having proven it can drive traffic to apps through the App Center and through ads placed in Facebook users' News Feeds, Facebook's now seeking to make money off of that traffic.

Here's a bonus: It keeps iPhone and Android developers, who might otherwise favor tools built into Apple and Google's mobile platforms, loyal to Facebook, since adding Facebook's social features to their apps is pretty much a requirement to get distribution—paid or unpaid—on Facebook.